The present invention relates generally to the field of photon detector arrays and acquisition of signals from such arrays. More particularly, the invention relates to a novel arrangement for extracting signals from a position-sensitive avalanche photodiode array such as for PET imaging.
A wide range of applications exist for photodiode arrays, particularly in imaging equipment. A variety of such photodiodes are known and are currently in use. For one particular type of photodiode, typically referred to as a position-sensitive avalanche photodiode (PSAPD), photons can be detected and located at positions where they impact the array. PSAPDs are currently used for medical and other imaging applications, such as positron emission tomography (PET) imaging. Their use is particularly of interest in such applications due to their ability to sense the position of photons impacting the array.
In PET imaging systems, an image is generated based upon the impact of radiation (generated by a nuclear decay event) with a scintillator. Gamma rays resulting from interaction of a positron and corresponding electron in a subject enter into the scintillator and are converted to photons that can be detected by optical sensors. For example, light emitted from a particular location in the subject may be detected using a PSAPD or other photo detector such as a photo multiplier tube (PMT).
PET detectors have been demonstrated that use dual-end readout arrangements, including PSAPDs and a fast single-channel PMT for simultaneously detecting data. In such arrangements, excellent timing resolution can be obtained for time-of-flight PET imaging, in addition to high spatial resolution and depth-of-interaction (DOI) capabilities. Such position, timing and energy information is generated for each gamma ray released in a subject and received by the scintillator.
PSAPDs used in experimental PET imaging systems have, however, been relatively small, such as on the order of 14 mm×14 mm. Such PSAPDs can be attached on one end of a scintillator array to provide position information, with the single-channel PMT on the opposite end to provide timing information. Energy information is determined by combining the signals from the PSAPD and the PMT, and the relative signal levels on the two detectors provide DOI information.
A typical PET imaging system includes a large number of such detector arrays, however. Thus, small PSAPDs such as those used in demonstrations require a large number of front-end electronic channels and time-intensive assembly and testing for practical applications. Moreover, when grouped into an actual imaging system, extraction of output signals from the PSAPDs requires relatively high density wiring, high-density electronics for processing signals, and so forth. Resulting operating temperatures can become elevated, leaving to degradation in the performance of the PSAPDs by increase in noise levels. Further improvements based upon these smaller sizes would appear to require reduction in temperatures well below room temperature, generally to be avoided in practical applications.
There is a need, therefore, for an improved position signal extraction technique from PSAPD arrays that avoids such drawbacks. In particular, there is a need for improved PSAPD arrays of larger size that can identify the location of incident radiation photons in a scintillator without substantially increasing the number of output channels for the sensed data.